The invention relates to a process for alkylating an isoparaffin with an olefin using HF acid catalyst.
The HF alkylation process itself is well known in the art. Work on this process began during World War II, and has continued today. Recent patents showing the trend of development of the HF alkylation process are U.S. Pat. No. 3,560,587 (Class 260-683.48), U.S. Pat. No. 3,755,485 (Class 260-671R) and U.S. Pat. No. 3,825,617 (Class 260-683.48) and U.S. Pat. No. 3,845,158 (Class No. 260-683.49), the teachings of which are incorporated by reference herein.
U.S. Pat. No. 3,560,587 shows a modern reactor design wherein olefin, isobutane, and acid are contacted in a reactor containing multiple feed injection points. The reactor contains provisions for cooling of the reaction mass. A reaction soaker is used. In this design, the olefin and isobutane reactants contact one another in the piping leading up to the reactor vessel.
In U.S. Pat. No. 3,755,485, the advantages of operating the alkylation process in the presence of a unique organic diluent, or catalyst diluent, formed by reacting a terpene with an alkylation catalyst, are disclosed. This patent is cited to show some of the recent work done with organic diluents. The function of this organic diluent is to modify or attenuate the catalytic activity of the HF acid phase in the reaction zone.
In U.S. Pat. No. 3,825,617, the hyperactive character of virgin HF acid catalyst is recognized. The invention disclosed in this patent is a way to "startup" a unit by quickly generating an organic diluent, in situ, by maintaining special conditions in the reaction zone, including low temperatures and low amount of isobutane recycle relative to olefin reactant. Again, the teaching is directed toward attenuating the activity of the HF acid in the reaction zone.
In my U.S. Pat. No. 3,845,158, I disclosed that use of organic diluents to attenuate the activity of the HF acid catalyst is also useful when applied to a fluorination step. The fluorination step is performed to improve the subsequent alkylation reaction.
Other examples of the use or organic diluents may be found in U.S. Pat. No. 3,408,419 (Class 260-683.51) and in U.S. Pat. No. 3,538,183 (Class 260-683.48), the teachings of which are incorporated by reference.
In all of these references the patentees attempt to modify the action of acid in the reaction zone. There is no explicit recognition of the problems that a refiner encounters if his recycle isobutane contains a free HF acid phase suspended therein. There is a problem, because the acid contained in the recycle isobutane stream is exceedingly pure, having been recovered as an overhead fraction in at least one fractionating column. The organic diluent normally present in the HF acid in the reaction zone, is no longer present in the acid phase because the fractionation step separated the acid from its diluent.
The way refiners have attempted to solve this problem in the past is to provide yet another stage of fractionation to remove HF acid from recycle isobutane. Another altenative is to redesign the existing HF alkylation product fractionation facilities to insure that no free HF acid phase will form in the recycle isobutane.
Many times it is not possible to modify an existing HF alkylation unit fractionation facilities without great capital expense. In the instances when modification of fractionation facilities is not economically justifiable, refiners have attempted to find other ways of ameliorating the bad effects of a free HF acid phase in recycle isobutane. In U.S. Pat. No. 3,775,510 the teachings of which are incorporated by reference, a recycle isobutane phase is cooled and passed to a two phase separator. The HF acid phase is removed and the recycle isobutane charged to the reactor. This will remove most of the dissolved HF acid in this stream. Such an approach adds significantly to the capital expense of a unit, and also consumes much utilities, and has not found great favor in the art.
Another attempt to solve this problem was disclosed in U.S. Pat. No. 3,818,938 (Class 137/604) the teachings of which are incorporated by reference. The patentee attempted to prevent the occurrence of undesirable side reactions between acid, in a recycle isoparaffin stream, and olefin feedstock, by providing a feed nozzle which would promote intense mixing of the feed components and minimize the chance that olefins would contact pockets of acid before dispersed in isobutane. Although the apparatus disclosed in this patent was an improvement over the prior processes and an improvement which could be incorporated into many existing units, without great incremental expense, it was an attempt to mechanically eliminate a problem caused by the chemical activity of the free acid phase.
Because none of the known methods of minimizing the deleterious effects of a free HF acid phase in the recycle isobutane stream charged to an HF alkylation reactor were completely satisfactory, for some units, work has continued to try to find a better way to operate the many HF alkylation units in existence today. With the trends toward unleaded gasoline in many countries in the world, refiners have been striving to improve the performance of all of their processing units. Of crucial importance to a refiner's effort to produce great volumes of high octane unleaded gasoline is the operation of the HF alkylation unit.